An international team of astronomers have detected an unexpected population of more than 20 blue straggler stars of similar age in young globular cluster NGC 2173. 

Star clusters have long been thought of as ‘infertile’ stellar systems which cannot form new stars, since all the gas from their birth environment has long been used up.

Only collisions or mergers of stars can rejuvenate old stars, in much the same way as a human might get a facelift. Such stars are known as blue stragglers because they appear to straggle behind their neighbours; they still resemble extremely hot (and therefore blue) young stars.

“In principle, stellar collisions or binary mergers should not take place at the same time. They will happen randomly in star clusters and produce blue straggler stars that seem to have different ages”, says Physics and Astronomy’s Dr Chengyuan Li, who led the research team.

However, the team detected a well-defined burst of blue straggler stars in this cluster, despite NGC 2173 being a relatively sprightly one to two billion years old.

Previously, astronomers have only found such bursts of blue stragglers in globular clusters 10 billion years old or older that had undergone a catastrophic core-collapse event, where the core of the cluster collapses under the gravity of all the stars in that small volume of space.

“This could suddenly trigger lots of stellar collisions, producing many blue straggler stars at the same time,” explains co-author and Macquarie colleague Professor Richard de Grijs.

“However, we did not find any evidence that NGC 2173 had experienced such a collapse event. In addition, the cluster isn’t dense enough for many stellar collisions to occur,” says Chengyuan.

“We suspect that there must be some hidden physics which suddenly produces so many blue straggler stars at roughly the same time.”

The study was published in The Astrophysical Journal in March, and featured in a Nature Astronomy News & Views commentary in May.

The research team included astronomers from Macquarie University’s Research Centre for Astronomy, Astrophysics, and Astrophotonics, the National Astronomical Observatories of the Chinese Academy of Sciences, the Kavli Institute for Astronomy and Astrophysics at Peking University, the Yunnan Observatories of the Chinese Academy of Sciences, and the Department of Astronomy of China West Normal University.

Banner image credit: Large Magellanic Cloud cluster NGC 2173 by Dr Licai Deng, HST/NASA/ESA